There has been a progressive development of very deepwater reservoirs of gas and/or oil around the world. Until about 10 years ago, very deep water was defined to be any depth greater than about 1000 m. Currently however, so many pipelines have been installed in depths greater than this that the definition of very deep water is currently about 2000 m. This development in installation capability is continuing and currently pipelines in 3500 m water depth are being planned.
The pipelines are typically installed empty, i.e. filled with air at ambient pressure and only filled with oil or gas under pressure once installation is completed. A major risk experienced during the installation of these deep-water pipelines is from the pressure applied by the water causing the pipe to deform out of its initial round shape and deform into an almost flat configuration. This is called external pressure collapse and if not controlled can result in the total loss of the pipeline. The dimensions, i.e. diameter and wall thickness and to a lesser degree the material properties, of a very deep-water pipeline are therefore determined by the potential for external pressure collapse.
This is in complete contrast to the design of a conventional shallow-water or onshore pipeline where the wall thickness is sized to resist internal pressure from the fluid it is to carry rather than external pressure.
Various theoretical studies of external pressure collapse have been carried out and numerical modelling has also been used to calculate the maximum water depth at which a pipeline with specified dimensions can safely be installed. However, the consequences of external pressure collapse buckling are so great that these theoretical studies are not sufficient for confident management of the risk. Also, the most important method for reducing the potential for such local collapse, by increasing the wall thickness of the pipe, is so expensive and possibly not technically realisable, that the proposed pipeline might well not be commercially feasible. This in turn raises the possibility that the exploitation of the gas or oil reservoirs are abandoned.
The alternative to basing all design on the results from theory is to additionally carry out tests. Indeed, several tests have been carried out for a range of pipe wall thicknesses. These tests involve placing long lengths of specially fabricated pipe in special pressure chambers and increasing the external pressure until collapse occurs. Only one or two laboratories have such facilities available and the tests are very expensive, in the order of $100,000 for one test.
Codes have been prepared to provide a basis for the calculation of the dimensions for pipes that are required to operate at specified great depths. These codes encompass safety factors that are intended to ensure that the natural variations in pipe dimensions and material properties that occur during the manufacture of a pipeline that could be 1000 km long will not undermine the capacity of the pipeline to withstand the external pressure without collapse occurring. However, the factors are based on the few previous available tests; the possibility of carrying out such tests on complete pipe joints during fabrication of the pipe are not realistic since the tests take a significant time to be set up and completed.
Only one joint of a pipeline needs to collapse to flood the whole line. It is therefore axiomatic that a long deep-water pipeline is more vulnerable to collapse than a short deep-water pipeline purely because there is a greater statistical probability in a long line of a single joint manufactured sufficiently out-of-specification to precipitate collapse. There is a direct analogy with “the weakest link in the chain” as regards pipeline failure due to external pressure collapse. Given that the codes of practice are based on the collapse test results of a small finite number of joints of line pipe, the design codes have to introduce a factor based on overall length to increase the wall thickness down the whole route simply to address the increased statistical exposure of a long line to a single fatally out-of-specification pipe joint.
There is thus a need for a test method that can replicate the effects of external pressure to cause the collapse of long pipelines and that is easy to set up and complete.
This invention is based on the recognition that the deformations that lead to external pressure collapse are uniform along the pipe and that therefore the occurrence of external pressure collapse will be the same for a ring cut from the pipe as for the complete joint length of pipe that is subjected purely to external pressure.